The present invention is directed to a lift truck-mounted load-handling clamp adapted for handling stacked loads of different sizes simultaneously, such as two stacked paper rolls of abbreviated length and different diameters. More particularly, the invention is directed to an improvement in the clamp disclosed in U.S. Pat. No. 4,682,931, which is hereby incorporated by reference, in order to facilitate the selection by the operator of different maximum magnitudes of clamping force to prevent damage to the loads from overclamping.
A common requirement in the paper industry is the handling of half-length paper rolls, which are normally handled by a lift truck roll clamp in pairs having different diameters, stacked one atop the other. Lift truck paper roll clamps specially adapted for handling such stacked rolls have been available in the past and normally consist of a pair of separately-actuated clamp arms on one side of the clamp, in opposed relation to a single, larger clamp arm assembly on the opposite side of the clamp. The separately-actuated arms give the clamp the ability to apply clamping force to two cylindrical objects of different diameters stacked one atop the other. Similar clamping capabilities can be useful with respect to other types of loads, such as stacked pairs of bales or cartons of different sizes.
The above-mentioned U.S. Pat. No. 4,682,931 provides a solution to a problem previously experienced by clamps of this type due to their inability to attain the required clamping force on one of the separately-actuated clamp arms without attaining it also on the other separately-actuated arm. For example, such clamp structures have the separately-actuated clamp arms powered by separate hydraulic cylinders connected in parallel to a source of pressurized fluid, requiring that the pressure buildup in the two cylinders during clamping be identical. The problem with such a structure is that, if only a single half-length roll or other load is to be handled, clamping pressure on the load-engaging arm cannot be attained until the other arm is closed to its maximum extent, which is very time-consuming. Conversely, on opening of the clamp arms to release a load, the release of both clamp arms is not usually simultaneous due to different frictional resistances in the respective arm mechanisms, sometimes requiring full opening of one clamp arm before the other can release sufficiently to disengage the load.
The above-mentioned U.S. Pat. No. 4,682,931 offered a solution to these prior problems by providing a flow regulator of the divider/combiner type which required the respective movements (or lack thereof) of the pair of clamp arms to be simultaneous until the regulator was overridden by an override assembly automatically in response to the attainment of a predetermined clamping force by one of the clamp arms, after which nonsimultaneous movement of the clamp arms was enabled by the override assembly.
However, a significant drawback to the system of U.S. Pat. No. 4,682,931 developed when it became important for clamping systems to have easily-operable selectively-variable clamping force adjustment systems, usually of the multipressure relief type, so that the operator could quickly select a different predetermined maximum magnitude of clamping force for each different load to prevent damage to the load from overclamping. The override assembly of the '931 patent, due to its automatic responsiveness to the attainment of a predetermined clamping force and its inability to change its responsiveness to accommodate higher or lower clamping forces without time-consuming adjustment, could not operate properly independently of the operator's alternating selections of different maximum clamping forces for each different load. For example, if the override assembly were adjusted to operate in response to the attainment by one of the clamp arms of a clamping force lower than the maximum force selected by the operator, the override assembly would operate prematurely to shunt pressurized fluid to the other clamp arm before the operator's selected maximum clamping force could be attained. Alternatively, such override assembly would not operate at all if it were adjusted to operate in response to a clamping force higher than the maximum force selected by the operator, since such clamping force would not be attained in view of the operator's selection.
Another drawback to the system of U.S. Pat. No. 4,682,931 was that, when opening the clamp arms, the maximum opening force attainable by each clamp arm was limited by the pressure limit of the divider/combiner flow regulator valve in its combining mode, unless the other clamp arm had been opened completely. This was because fluid pressure tending to open the clamp arms in parallel was limited to that which opened the clamp arm having the least resistance to opening. If it were desired to push a load with the back of a first clamp arm, referred to as "backhanding" the load, while the second clamp arm encountered no such loading, the only resistance to opening of the second clamp arm, unless it were fully opened, was that imposed by the pressure of the fluid exhausted from the second clamp arm's fluid actuator during opening. Such exhaust pressure was dependent upon the pressure limit of the flow regulator.